1. Field of the Invention
The present invention relates to a method for controlling an actuator, and more particularly to a method for detecting the saturation status of non-synchronous incremental actuators using a variable position estimate window.
2. Description of Related Art
An actuator is a mechanical device used to move or control another device. For example, an air conditioning and heating control system may use an actuator to control a damper in an air chamber so that the damper can act as a throttle and control air flow through the chamber.
Due to their relative low cost, the current trend in the actuator industry is to use non-synchronous actuators. Non-synchronous actuators are controlled using three wires--positive, negative, and ground--whereby a controller controlling the actuator can move the actuator in a first or second direction depending upon whether a drive voltage is applied between the positive and ground wires or the negative and ground wires.
Non-synchronous actuators, however, move at different speeds in different directions, due, in part, to different loading on the actuator. Moreover, this speed-disparity is not constant; it can and often does change over time with the actuator's age, fatigue, and other factors. As a result, it is not uncommon for an actuator to move faster in a first direction than it moves in a second direction, or for an actuator, moving in a first direction for a given period of time and then in an opposite second direction for the same period of time, to not return to the same position it was in prior to moving.
As discrepancies accumulate over time, the controller's estimated positions of the actuator quickly become unsynchronized from their actual positions. And after a short period of operation, the controller controlling the actuator may estimate that the actuator is at mid-stroke when the actuator is actually at an ultimate end-stop.
One of the current methods for dealing with speed-disparity is called "overdrive," whereby the controller continues to drive the actuator even if the controller estimates that the actuator is "saturated" (at an end-stop). If the actuator is genuinely saturated, overdriving forces the actuator to continue to drive against the end-stop, which results in heat build-up, thereby shortening the effective life of the actuator. If the actuator is not saturated, the overdrive will force the actuator to drive to an end stop, causing unacceptable control disturbances. While the tolerance for overdrive is reasonably high, the amount of overdrive needed to maintain synchronization with current non-synchronous actuators is significant. As a result, present actuators often settle at a partial stroke position when the controller estimates they are fully open or closed.
While control algorithms known in the art estimate the position of the actuator based on its "on" and "off" time and direction, these algorithms assume a constant actuation speed based on a nominal stroke time of the actuator. These algorithms are unsatisfactory for actuators with different stroke times that move at different speeds in different directions.
Furthermore, when a controller controls an actuator incrementally (i.e., moving the actuator by discrete increments in a specified direction relative to the current position of the actuator, as opposed to moving the actuator to an absolute position), the controller remains unable to accurately determine the actual position of the actuator. The controller must determine the actual position of the actuator so that it can disable movement of the actuator if the actuator is saturated. Thus, continual and accurate estimation of the position of the actuator, in anticipation of detecting a saturated condition and acting thereupon, is of paramount importance in controlled operations.
As is evident from the foregoing, a need exists for a method of minimizing the amount of error, time, and overdrive required to accurately detect the saturated condition of a non-synchronous incremental actuator.